Double clad active optical fibers are widely used in fiber lasers and amplifiers. Fiber lasers and amplifiers with a CW output power of up to several kilowatts have been demonstrated. Besides to the obvious advantages the known double clad active optical fibers have serious shortcomings.
Firstly, there is a limitation on the level of pump power that can be launched into a double clad fiber. This consequently limits the power scaling capability of fiber lasers and amplifiers. Furthermore in single mode fibers the fundamental mode propagation requirement imposes restrictions on the largest core diameter. This requirement dictates that even for fibers with a small numerical aperture (NA<0.07) a core diameter should not exceed 12 μm for an operational wavelength of 1 μm. At the same time, since pump absorption in a double clad fiber depends on the core/cladding diameter ratio, the outer diameter of a double clad fiber should not exceed 300-400 μm. Taking into account the limited brightness of available pump sources, the limitation in the fiber diameter automatically leads to a limitation in the output power of an active component.
Secondly, the pump absorption coefficient for each cladding propagated mode is determined by the overlap between the mode field distribution and the dopant (e.g. rare earth ions) distribution in the core. As a result, each mode will be absorbed with different efficiency due to a difference in the mode field distribution of various modes. So, virtually, it is possible to divide all cladding propagated modes into two groups—“absorbable” and “unabsorbable” modes.
The first group of modes has an axially symmetrical mode field distribution. These are the modes with a maximum intensity at the doped core, i.e. in the centre of the fiber and, as a result, these modes are well absorbed and contribute very much to light amplification. The second group of modes, that still contains an essential fraction of the pump power, has a small overlap integral with the core and the dopants. Therefore the modes in this group are not absorbed efficiently in the core of the fiber and do not notably contribute to light amplification.
In terms of ray optics, the “absorbable” modes can be understood as meridian rays which propagate along the fiber crossing an optical axis of the fiber i.e. the doped core. The “unabsorbable” modes can be understood as skew rays. These skew rays have a spiral trajectory and are still guided by the inner cladding of the double clad fiber, but propagate without crossing the doped core.
Typically pump absorption occurs according to the following scenario. The first group of modes, the meridian rays, are absorbed very quickly as they propagate along the length of the fiber. The rest of the pump radiation concentrated mostly into the skew rays propagates practically without any absorption. The modal spectrum for pump radiation before and after propagation through a double clad fiber is shown in FIG. 1. Since the different modes have significantly different absorption the modal spectrum changes dramatically as light propagates along the fiber; spectral “holes” are burned into the spectrum. The modal spectrum begins to stabilize after the “absorbable” modes have undergone significant absorption.
The non-uniform distribution of population inversion along the fiber length is largely a result of the change in modal content of the pump radiation with propagation distance. The part of an active fiber which has an insufficient inversion, works as an absorber. This causes deterioration in the pump conversion efficiency of lasers and amplifiers.
There are three main approaches for improving the limited pump absorption in a double clad fiber. The first one uses fibers with a special shape of cladding. The cross-section of the cladding can be e.g. truncated, double truncated, rectangular, hexagonal or decagonal and the fiber can have a core offset from the middle of the fiber (FIG. 2). The special shape of the cladding or an offset core enriches the modal spectrum as the rays' propagation trajectories become more chaotic. This makes the modal spectrum of the fiber more continuous. As a result, a bigger part of the total pump power is concentrated in the “absorbable” modes of the modal spectrum compared with a circular symmetric fiber. The reason for low absorption of the pump power into the active core is the regularity of the fiber, i.e. the fact that the mode spectrum is the same for all parts of the fiber. It is known from prior-art that a special geometry of the cladding leads to an increase in the pump power absorption, but by no means completely eliminates the problem of saturation of the absorption.
The second approach exploits non-regular bending of the fiber introduced by granular matter such as sand, metal, ceramic or plastic particles embedded into the fiber coating. The periodic non-regular stresses and bending cause a mode coupling, which, in turn, leads to transfer of part of the optical power from skew rays (“unabsorbable” modes) to meridian rays (“absorbable” modes) (FIG. 3a). Although, the non-regularity may improve the absorption of pump power, this effect strongly depends on the fiber geometry. As it is known, the mode coupling coefficient D is strongly dependent on the outer diameter of the fiber:D∝d8/b6λ4,  (1)where d is the core diameter, b is the fiber's outside diameter and λ is the operational wavelength.
As we can see from (1), the mode coupling coefficient for fibers with a large diameter (300-500 μm) is extremely low. Furthermore this method is applicable only for relatively thin fibers with an outer diameter of cladding of less than 200 μm, because it is difficult to bend a 300-600 μm diameter fiber with the required spatial period. Additionally, a chaotic mechanical stress and perturbation of cladding may lead to the coupling between skew and leakage rays resulting in an increase in the loss of pump power. This would lead to deterioration in the efficiency of an active component.
The third approach for improving pump absorption in double clad fibers has been disclosed in U.S. Pat. No. 6,687,445 B2 where special “truncated regions” or “filaments” are embedded into the fiber cladding (FIG. 3b). The “truncated regions” could be made of glass, air, ceramic, metal or other materials. The “truncated regions” act as scattering centers, which enhance the conversion of “unabsorbable” skew rays to “absorbable” meridian rays. This method has an obvious disadvantage as scattering at the “truncated regions” embedded directly into the cladding will inevitably lead to significant leakage of the pump power out of the fiber. I.e. the method will result in pumping losses.